Supersonic Jets Fixed the Sonic Boom… So Why Is the Sky Still the Problem?

It was expected that supersonic travel could halve long-distance flights, allowing one to cross oceans in just a matter of hours. However, this dream of supersonic travel was soon forgotten after the Concorde was retired, primarily because the thunderous sonic boom resulting from flying faster than the speed of sound was deemed impossible over land due to public outcry.

However, modern aerospace engineers are trying to revive supersonic travel with new planes that are able to produce much quieter shockwaves.


Additionally, what scientists are finding out is that perhaps the biggest problem with supersonic travel may not be with the plane itself, but with the atmosphere above the cities that hear these shockwaves. A recent study has found that temperature layers, winds, and humidity within the atmosphere can change shockwaves as they travel from an airplane down to the ground.

Understanding Sonic Booms

A sonic boom forms when an aircraft travels faster than the speed of sound, compressing the air into a series of shockwaves that trail behind it. When these waves reach the ground, they create a rapid pressure change that people perceive as a loud boom.

Traditional supersonic aircraft produced extremely intense shockwaves. According to studies published in the AIAA Journal, the sonic booms generated by earlier supersonic aircraft often reached levels of 105-110 decibels, comparable to the noise level of a loud rock concert.

Modern aircraft concepts aim to reshape the pressure wave so it spreads over a longer distance and reaches the ground as a softer “thump” rather than a sharp bang. Engineers working on the X-59 QueSST are targeting a sound level of about 75 decibels, which is closer to the sound of a car door closing.

However, even when aircraft are designed to produce quiet shockwaves under ideal conditions, atmospheric changes can dramatically alter how those waves behave before reaching the ground.

Temperature Layers Bend Shockwaves

The atmosphere is not a uniform layer of air. Temperature changes with altitude, creating layers that affect how sound waves travel through the atmosphere.

According to research on shockwave propagation published by the Royal Society, temperature differences can bend the path of sonic booms through a process known as atmospheric refraction. In some situations, the shockwave curves upward away from the ground, creating quiet zones where people hear nothing at all. In other situations, the wave bends downward and becomes concentrated in a smaller area.

These temperature effects depend strongly on the vertical structure of the atmosphere above a city. A cooler upper layer combined with warmer surface air can cause sound waves to travel farther and strike the ground more strongly than predicted by simplified models.

Winds Can Redirect the Boom

Wind patterns across different altitudes also influence the path of sonic shockwaves. Air currents can act like moving channels, redirecting sound waves as they travel through the atmosphere.

Studies using atmospheric forecast data from the Global Forecast System show that wind shear can modify the spread of a sonic boom as it moves toward the ground. According to modeling described in the AIAA Journal, yearly variations in wind profiles can change the strength of a sonic boom by as much as 10 decibels along the same flight path.

NASA research on sonic boom propagation explains that wind and temperature profiles together determine the exact path that the shockwave follows. The agency notes in its technical reports that “atmosphere temperature and wind velocity profiles dictate the propagation path,” meaning that the same aircraft can produce different noise levels on different days.

Humidity Alters How We Hear It

Humidity introduces another layer of complexity. Water vapor affects the speed of sound in air and also changes how sound energy is absorbed.

Research summarized in studies of atmospheric acoustics shows that humid air tends to soften the sharp edges of a shockwave by absorbing higher frequencies. However, this effect can sometimes make mid-range frequencies more noticeable to the human ear.

Aerospace analysis published in AIAA indicates that relative humidity at cruising altitudes plays a measurable role in determining how loud a sonic boom sounds on the ground. Seasonal changes between dry winter air and humid summer air can shift perceived sound levels by several decibels.

Why Ground Testing Matters

Given the varied conditions, however, scientists cannot simply rely on lab tests and computer simulations to understand how these new supersonic planes will affect the real world. Instead, they must undertake extensive tests of these planes as they fly over populated areas to see how these sonic booms behave in real-world conditions.

NASA’s quiet boom research program aims to undertake these kinds of measurements by using microphones placed over populated areas. These measurements will also enable regulators to understand how often these kinds of noises are heard by residents in these areas during regular flights.

As explained by Domenic Magliacane, a sonic boom researcher with NASA, in reports about their research program, the design of these kinds of planes is only one part of the equation. Measurements must also be taken to understand how quiet these kinds of supersonic planes will be.

Ultimately, the ability of new kinds of supersonic planes to succeed may depend as much on understanding the constantly changing conditions of the atmosphere that surrounds any given city.